A correlation: 19th century cotton production and Obama votes

This is an overlay of two maps. One is the 2008 presidential election results and the other is cotton production in 1860 (each dot represents 2000 bales). Strange Maps writes:

The link between these two maps is not causal, but correlational, and the correlation is African-Americans. Once they were the slaves on whom the cotton economy had to rely for harvesting. Despite an outward migration towards the Northern cities, their settlement pattern now still closely corresponds to that of those days.

I wonder why the correlation breaks down in the cotton-heavy region in southern Tennessee, just north of the Alabama border.